Improve the comments for RE2::FullMatch() et al.
Improve the comments for RE2::FullMatch() et al.

Change-Id: Ibf2c9d9282988fbb30c04074c3ba0701b58e562d
Reviewed-on: https://code-review.googlesource.com/c/re2/+/57790
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

google/re2,google/re2,google/re2,google/re2,google/re2